This invention relates to lubricating oils containing a VI improving, hydrogenated, high molecular weight copolymer of a diolefin and a lower alkyl methacrylate or lower alkyl acrylate. Incorporating a higher alkyl methacrylate in the monomer charge of the copolymer provides a pour point depressant, VI improver for lubricating oils. Graft polymerizing the hydrogenated copolymer with a polar, nitrogen-containing graft monomer provides a dispersant, VI improving additive for lubricating oils and a dispersant for hydrocarbon fuels.
Various additives have been proposed to impart beneficial properties to lubricating oils and hydrocarbon fuels. These properties include enhanced viscosity, viscosity-temperature relationships, dispersancy, pour point depression, oxidation inhibition, anti-rust, and anti-wear properties. The ability of a lubricant to provide adequate flow at low temperatures and good film thicknesses at high temperatures is referred to as a good viscosity-temperature relationship. Dispersancy refers to the ability of a lubricant to prevent settling of sludge or deposits which are formed by the direct oxidative degradation of the lubricant or as the result of complex reactions of gasoline and blowby gases in automotive crankcases. If these deposits are not dispersed in the lubricant they will settle out and cause the plugging of filters or possibly the sticking of moving parts where tight tolerances are involved. In addition, dispersants are added to gasoline and middle distillate fuels, such as home heating oils, diesel fuels, and jet fuels because these fuels tend to deteriorate oxidatively upon standing to form gummy deposits. In the case of gasoline, such gummy residues are deposited in the carburetor, making control of air-fuel ratio impossible.
The major types of conventional lubricant additives include polymethacrylates, olefin copolymers, and hydrogenated diene-styrene polymers.
Polymethacrylates exhibit excellent viscosity-temperature relationships in lubricating oil as a result of their differing solubility in lubricating oil at high and low temperatures. Polymethacrylate additives may be modified to impart dispersancy by graft polymerizing polar, nitrogen-containing monomers onto the polymethacrylate backbone.
Olefin copolymers, such as ethylene-propylene copolymers, are more efficient thickeners for lubricating oil than polymethacrylates. The improved thickening efficiency allows them to be effective at lower concentrations in lubricating oil than polymethacrylates. The improved thickening efficiency of olefin copolymers is a function of the high percentage of their molecular weight in the polymer backbone as compared to polymethacrylates which have a lower percentage of their molecular weight in the backbone and a higher percentage of their molecular weight in the side-chain alkyl groups. Olefin copolymers may also be modified, by a different graft polymerization process than that used to graft polymerize polymethacrylates, to incorporate a dispersant functionality onto the olefin copolymer backbone. While olefin copolymers are excellent thickeners, they do not possess the excellent viscosity-temperature relationships of polymethacrylates. In addition, olefin copolymers do not exhibit pour point depression, and the addition of sufficient quantities of conventional pour point depressant additives to the olefin copolymer may result in an incompatible lubricating oil concentrate.
Hydrogenated diene-styrene copolymer additives are also efficient thickeners for lubricating oil but are more oxidatively unstable than polymethacrylates and do not possess the excellent viscosity-temperature relationships of polymethacrylates.
Accordingly, it would be apparently advantageous to combine the beneficial properties of polymethacrylates with the beneficial properties of olefin copolymers to create a lubricating oil additive having the best features of each type of additive. Physical mixtures of polyolefins and polymethacrylates are, however, incompatible. Mixtures containing more than about 5 weight percent polyolefin separate into two phases in lubricating oil and this incompatibility, in most instances, renders simple physical combination ineffective as an oil additive.